Interactions of adsorbed organic layers with hydrogen atoms on platinum electrodes

Abstract

Coverages of smooth Pt foils with H-atoms in presence of CO and “reduced CO₂” have been investigated in 1 M H₂SO₄ at 40°C. Compared with H—H interactions, H—CO interactions are repulsive above 0·15 V (R.H.E.). When θ_CO is expressed in terms of adsorbed CO molecules, there is apparently no net interaction for potentials ⩽ 0·15 V. When θ_CO is expressed in terms of sites occupied, the H—CO interaction is repulsive at all potentials. The H—“CO₂” interaction is repulsive above 0·20 V but attractive below this potential.

The data suggest that the “uniform heterogeneity” model of H-adsorption on Pt is inadequate and that there are at least two distinct kinds of H-atom sites. Within these adsorption regions, the “heterogeneity” cannot be explained solely on the basis of an a priori site energy distribution; on type II (“strongly bonding”) H-sites, the observations are consistent with “induced” heterogeneity. The H—“CO₂” interaction at high hydrogen coverages must result from lateral interactions, probably hydrogen-bond formation. Because of these interactions, H-atom charging methods of estimating organic coverage on Pt must be used with caution. The cathodic method is least suspect. Although “conventional” electrochemical techniques show them to be identical, their interactions with H-atoms prove CO_ads and “reduced CO₂” to be different species.